The instant invention relates to electrophotocopying machines in general and more particularly to rollers used for charging the photoconductor and transferring the developed image from the photoconductor to a receiver sheet in such machines.
Numerous prior art teachings in the field of electrophotographic copying disclose various methods and devices for charging the surface of a photoconductor so as to obtain a latent image from an original document on the photoconductor and for transferring the developed image from the photoconductor to a receiver sheet such as a plain copy sheet. To enable the development of the latent image on the photoconductor and the transferring of the latent image to a receiver sheet, several stations are arranged in proximity to and cooperate with the photoconductor to perform certain functions. At the charging station the photoconductor is charged to a selective polarity, be it positive or negative. The photoconductor then moves to the exposing or imaging station where a latent image is copied from the original document. Next, the electrostatic latent image is developed at a developer station to form a toned image on the photoconductor. The toned image is then transferred from the photoconductor to a receiver sheet at the transferring station. To complete the cycle, the photoconductor is erased and cleaned preparatory to another cycle.
One of the commonly employed techniques of applying an electrostatic charge on the photoconductor and of transferring the developed image is through the use of a corona discharge system. Among the disadvantages of the corona are the generation of appreciable amounts of ozone, the high voltages required, the high power required, the variability of charge applied as a function of ambient conditions and in particular as a function of humidity, and the substantial amount of servicing required for the corona. For these reasons, it is known to substitute for the corona two conductive rollers, one of which has an insulating sleeve on it. A voltage is applied between the two rollers and results in a small current flow across the high resistance sleeve, thereby applying an electrostatic charge to the paper passed between the rollers. These rollers are in contact with one another and for this reason are generally used with coated copy papers which double as the photoconductor and receiver sheet and which can be fed between the rollers. A pair of conductive rollers obviously is not suited for use with plain paper copiers wherein the photoconductor is mounted on a drum or comprises an endless web.
Perhaps one of the most pressing problems associated with conductive rollers is the fact that the charging and transferring functions are performed by separate pairs of rollers at separate stations. Another problem relating to the separate processing stations is the fact that each of the separate pairs of rollers requires a separate power supply. With this type of design, the cost of a plain paper electrophotocopying device is relatively high, due to the individual cost of each roller and power supply. Accordingly, any reduction in the number of rollers and power supplies will reduce the cost of the device.
The foregoing problems are overcome by the instant invention which provides only a single conductive roller which is capable of both charging a photoconductor and transferring the toned image from the photoconductor to a plain copy sheet. The invention is particularly applicable for image retention on the photoconductor when it is desired to make multiple copies on plain paper of a single original document.